Transmission control devices, for example, like those used in some car navigation systems have been known which, by being connected to a cellular phone either wiredly or wirelessly, control the cellular phone and make it transmit a call to a predetermined telephone number. Such transmission control devices are normally used as hands-free devices in combination with a microphone and a speaker equipped in a vehicle. A transmission control device which is wirelessly connected to and controls a cellular phone is described, for example, in JP-A-2003-102058.
To have a cellular phone connected to the transmission control device, the ID of the cellular phone is registered in advance with the transmission control device. When an ID signal is received from the cellular phone, the transmission control device can determine that the cellular phone is the one registered with it. Based on the determination, the transmission control device establishes communications with the cellular phone.
In recent years, there are many people carrying plural cellular phones which include, for example, cellular phones provided by companies they work for and their privately-owned cellular phones. In such cases, they use different cellular phones for different call destinations. For example, they will usually use a privately owned cellular phone to make a private call and a company-provided cellular phone to make a business call.
Therefore, when using a hands-free device, too, it is necessary for them to use different cellular phones for different call destinations. Hands-free devices which have been in use are, however, either incapable of being connected to plural cellular phones or, even if they can be connected to plural cellular phones, capable of being connected to the plural cellular phones only according to a predetermined priority order, for example, according to the order of registration.
Therefore, when changing the cellular phone to be controlled by a hands-free device, it becomes necessary to change wire connections in a case where wired connections are used or to change connection settings in a case where wireless connections are used. In either case, troublesome work is involved. Requiring a vehicle driver to perform a procedure to change the cellular phone to use while driving is also quite problematic from a safety point of view.